1. Field of the Invention
This invention relates generally to wireless communications and, more particularly, to the operation of a Radio Frequency (RF) transceiver within a component of a wireless communication system.
2. Description of the Related Art
The structure and operation of wireless communication systems are generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e., operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with these operating standards, equipment interoperability is achieved.
In a cellular system, a regulatory body typically licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscriber units within the service area. Typically, these channels are equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that maybe caused by transmissions on a particular channel.
In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., Global System for Mobile Communications (GSM) cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies, and reverse link communications (from subscriber units to the base station) on a second set of carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carriers within the licensed frequency spectrum. In most, if not all, cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized and system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results.
Both base stations and subscriber units include RF transceivers. Radio frequency transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. Similarly, the RF receiver receives an RF signal, down converts the RF signal to an IF signal and then converts the IF signal to a baseband signal. In other systems, the received RF signal is converted directly to a baseband signal.
In the initial signal processing stages of an RF receiver, the received RF signal is converted to baseband through one or more steps. Initially, the received RF is mixed with a local oscillator (LO) to down convert the carrier frequency of the RF. It is also common to utilize a low pass filter coupled to the output of the mixer to remove introduced interferences. One problem with using a mixer and low pass filter, however, is that both add a fixed amount of gain to the received signal, irrespective of the received signal strength.
This amplified signal is passed to the baseband processor, which is sensitive to excessive signal strength which can cause quantification errors in downstream analog-to-digital converters (ADCs) and saturation in the output devices. Previous designs have included a programmable gain amplifier (PGA) between the low pass filter output and the baseband processor input to limit the gain when the received signal strength is excessive. This approach, however, requires a method to detect the peak amplitude of the received signal then provide a control signal to the PGA that is proportional to the received signal amplitude.
Since a received signal is comprised of a differential I and Q signal, the traditional method of detecting the peak amplitude is to take the square root of the sum of I squared and Q squared [(I2+Q2)1/2]. The square root function is traditionally performed using a digital signal processor (DSP) in the digital domain. Generally, though, it is advantageous to perform such peak amplitude detection in the analog domain and to perform gain control based upon the detected peak values. There is a need in the art, therefore, for an analog circuit and a method to adjust the gain level of the PGA according to the determined peak amplitude of the amplified signal.